Active Thermal Detection Could Broaden Use of Superresolution Techniques

Be it light, sound, or an electromagnetic wave, a probe beam operates the same way — the beam gets sent ahead and a wave of the same nature is reflected back. The absorbed energy is converted to heat, for which there has been no perceived use until now.

Researchers at the Institute for Basic Science (IBS) theoretically showed that the temperature increase caused by a probe beam could be used to generate a signal for detecting objects. Further, this so-called active thermal detection could enable superresolution imaging at all scales, said the researchers, compared to conventional superresolution techniques, which are limited to microscopy applications.

“Nobody tried to use thermal radiation for superresolution, even though this signal is so noticeable that it cannot be missed,” professor Francois Amblard said. “Our first and deceptively simple idea is to detect objects with their obvious signal, the thermal radiation.” When an object is illuminated by a probe beam with enough energy to cause its temperature to jump, the thermal radiation it emits is intense.

(a) Two objects are illuminated by a scanning focused energy source with a size larger than the objects or the distance between them. (b) The thermal light emission produced by the scanning illumination and the heating of the objects is spatially compressed compared to a linear response to the illumination. Courtesy of IBS.

The researchers theoretically verified the superlinearity of thermal radiation. They gave an exact quantification of the number of photons emitted by a heated object and showed that even a small temperature increase resulted in a significant change in the light emission. This phenomenon, together with active heating and a detection scheme, could be used to detect objects at a very high resolution.

The superresolution factor can be arbitrarily cranked up if a sufficiently high temperature is reached. “Our theory predicts that the emission spatial profile can be made arbitrarily narrow, leading to an improved localization of objects, and even in principle to an arbitrarily large superresolution,” researcher Guillaume Graciani said. “One expects then to be able to better resolve two nearby targets, or to better detect the shape of a target.”

The IBS study presents thermal radiation and its intrinsic superlinearity as a universal way to superresolve objects at all scales, from microscopic imaging to large flying objects such as airplanes. “Active thermal detection” could be used in thermal imaging for nondestructive testing, in lidar and radar technologies for self-driving cars, and for mid- or long-range detection of stealth objects. It also could lead to a new field of applications for state-of-the-art thermal photodetectors, such as superconducting nanowire single-photon detectors or HgCdTe avalanche photodiodes. A new type of thermal probe could be designed for superresolved thermal detection or imaging at microscopic scales.

The research was published in Nature Communications (www.doi.org/10.1038/s41467-019-13780-4).